It is conventional and routinely accepted in the art that the easiest route for producing tertiary aminocarbonates is to transesterify diethyl carbonate (DEC) with the appropriate alkanolamine in the presence of wide variety of basic catalysts. This route involves straightforward processing and provides reasonably good yields (i.e., &gt;80%). The route suffers from the high price of DEC, the limited sources of DEC, uncertainties about the future availability of DEC, and the burden of certain government regulations associated with handling the by-product ethanol.
In particular, DEC is manufactured almost exclusively by reacting ethanol with phosgene using technology developed over 50 years ago. Phosgene is an extremely toxic and corrosive chemical requiring great care in preventing releases. While the by-products from the phosgene-based manufacture of diethyl carbonate are not as toxic as phosgene, they are nevertheless hazardous requiring potentially troublesome disposal. Because of these environmental issues there are pressures to curtail the use of phosgene in the manufacture of intermediates. Most phosgene manufacturers are currently cautious about building new facilities especially for specialty uses such as diethyl carbonate. Since the use of DEC in making tertiary aminocarbonates requires the handling of ethanol first as a raw material in the manufacture of the DEC and then as a by-product from the tertiary aminocarbonate process, there are additional costs associated with the government requirements associated with handling ethanol.
The preparation of bis[(N,N-dialklyamino)alkyl]carbonates by transesterifying diethyl carbonate with tertiary alkanolamines in the presence of anhydrous potassium carbonate catalyst is known from U.S. Pat. No. 2,691,017. The bis[(N,N-dialklyamino)alkyl]carbonates thus produced were further reacted with various inorganic and strong organic acids to form salts which are non-toxic in therapeutic dosage. The yields of the tertiary aminocarbonates were not disclosed.
The use of bis(2-dimethylamino-ethyl)carbonate (DDC) as a catalyst for both the alcohol-isocyanate reaction and the water-isocyanate reaction in the formation of polyurethane foam is discussed in Willeboordse et al., J. Cellular Plastics 1(1) 76-84 (Jan, 1965). However, details of the preparation are not disclosed.
Patent application DE 4203908-A1 describes diaminocarbonate compounds of the formula R1--OC(.dbd.O)--OR2 where R1 and R2 are tertiary amino groups with methyl or C.sub.2-20 alkyl, phenyl, or C.sub.1-20 alkyl substituted phenyl substituents for use as catalysts in the production of polyurethane and polyurea having reduced odor. In one example, DDC was prepared by reacting 1.26 g-mole of dimethyl carbonate with 2.52 g-mole of dimethylethanolamine in the presence of potassium hydroxide catalyst. The by-product methanol was removed from the reaction mixture by azeotropic distillation with cyclohexane. The yield of DDC was 35%.
U.S. Pat. No. 4,324,739 indicates that (dimethylaminoalkyl)carboxylic acid esters are especially suitable for use as amine curing agents for polyepoxide compounds because they impart longer curing times which results in good processing properties, especially when the mixtures are used as adhesives. In one example, DDC was prepared by reacting 1 g-mole of diethyl carbonate with 2 g-mole of dimethylethanolamine using potassium hydroxide as the catalyst by distilling the by-product ethanol overhead through an 80 cm packed distillation column. The amount of DDC obtained indicates a DDC yield of 22% based on the starting DEC.
U.S. Pat. No. 5,214,142 describes a process for preparing aminoethers by decarboxylating the corresponding aminocarbonates using a suitable metal oxide catalyst at elevated temperatures. A number of examples in this patent are concerned with the decarboxylation of DDC to form bis(2-dimethylaminoethyl)ether, a commercially available polyurethane catalyst highly valued for its selectivity in catalyzing the water-isocyanate "blow" reaction. The patent describes the preparation of DDC by reacting 1 mole of diethyl carbonate with 6 moles of dimethylethanolamine (three times the stoichiometric requirement) using sodio 2-dimethylaminoethoxide as the catalyst in a reaction flask outfitted with a distillation column suitable for removing the ethanol by-product while retaining the diethyl carbonate in the kettle. The patent indicates DDC was isolated at a purity of 98.2% and a yield of 73.8% based on the starting diethyl carbonate. Dimethyl carbonate was also used as a starting material, but the resulting yields of aminoether were poor.
It has been discovered that dimethyl carbonate (DMC) can be used to manufacture tertiary aminocarbonates by partially transesterifying the DMC with a suitable alcohol (ROH) to form an intermediate carbonate MeOC(O)OR. This intermediate carbonate can then be used in a transesterification (TE) with the appropriate tertiary alkanolamine to form the desired tertiary aminocarbonate. By judicious use of processing aids (such as cyclohexane for breaking the methanol/DMC azeotrope) and recycling of the ROH the tertiary aminocarbonate yields achieved are comparable to those obtained when DEC is used.
The use of (DMC) rather than DEC for manufacturing these tertiary aminocarbonates is desirable because of DMC's greater supply options. DMC is manufactured not only from phosgene, but can also be manufactured by (1) liquid-phase oxidative carboxylation technology (U.S. Pat. No. 4,318,862) as currently practiced by EniChem Synthesis, (2) vapor-phase oxidative carboxylation technology (European Pat. No. 425,197) as currently practiced by Ube Industries, and (3) cosynthesis with ethylene glycol by reacting methanol and ethylene carbonate in the presence of either a heterogeneous catalyst (U.S. Pat. No. 4,691,041) or a homogeneous catalyst (U.S. Pat. No. 4,734,518). These technologies are either not appropriate for making DEC or the practitioners of the technologies have not seen the incentive for building facilities suitable for making DEC.
Although the oxidative carboxylation routes employ toxic carbon monoxide as a reactant, the hazards of these processes are significantly lower than those associated with phosgene. Because of the increased competition in the marketplace the price of a mole of DMC has recently been about half that of a mole of DEC for purchases in excess of 100 tons per year. Unfortunately DMC reacts with tertiary amines to give the corresponding quaternary ammonium salt with methyl carbonate as anion (EniChem Synthesis SpA, Dimethyl Carbonate Product Bulletin p. 9, 1992). In the case of DMEA these materials decompose to form undesirable by-products when the transesterification is attempted in batch equipment and the yield to DDC is typically less than 50%. DEC also incurs a transportation penalty relative to DMC since one mole of DEC weighs 31% more than one mole of DMC.
Thus, for economic reasons it is desirable that tertiary aminocarbonates (such as bis[2-dimethylaminoethyl)carbonate) be manufactured from the appropriate alkanolamine through a transesterification with dimethyl carbonate (DMC). However, because of the by-product reactions, tertiary aminocarbonates have most successfully been prepared by the transesterification of diethyl carbonate (DEC) with the alkanolamine. DEC works well, but currently costs about twice as much per mole as DMC.
However, it has been discovered that DMC can be used to manufacture tertiary aminocarbonates in batch equipment by partially transesterifying the DMC with a suitable alcohol (ROH) in the presence of a suitable catalyst to form an intermediate carbonate MeOC(O)OR. This intermediate carbonate can then be used in a transesterification with the appropriate alkanolamine in the presence of a suitable catalyst to form the desired tertiary aminocarbonate. By judicious use of processing aids (such as cyclohexane for breaking the methanol/DMC azeotrope) and recycling of the ROH the tertiary aminocarbonate yields achieved are comparable to those obtained when DEC is used.